Method of producing chromium containing alloys

ABSTRACT

A chromium-containing alloy, having very low contents of nitrogen and carbon, is produced by refining with a small but effective addition of aluminum, or an aluminum alloy, preferably an aluminum-lithium alloy.

United States Patent Grunbaum et al.

[451 May 27, 1975 METHOD OF PRODUCING CIIROMIUM CONTAINING ALLOYSInventors: Gunnar Grunbaum; John Erik Wallen; John Kjell Gustafsson;Gustaf Widmark, all of Sandviken,

Sweden Assignee: Sandvik Aktiebolag, Sandviken,

Sweden Filed: Dec. 21, 1973 Appl. No.: 427,094

Foreign Application Priority Data Dec. 29, l972 Sweden 17l46/72 US. Cl75/58; 75/1305 Int. Cl C2lc 7/04; C22c 39/!4 Field of Search 75/58,130.5

Primary Examiner-C. Lovell Assistant ExaminerPeter D. RosenbergAttorney, Agent, or FirmPierce, Scheffier & Parker [57] ABSTRACT Achromium-containing alloy, having very low contents of nitrogen andcarbon, is produced by refining with a small but effective addition ofaluminum, or an aluminum alloy, preferably an aluminum-lithium alloy.

6 Claims, 2 Drawing Figures METHOD OF PRODUCING CHROMIUM CONTAININGALLOYS The present invention relates to a method of producingchromium-containing alloys with low contents of nitrogen and carbon.

In ferritic chromium steels normal contents of carbon or nitrogen causematerial problems such as lowered impact strength, a high(ductile-to-brittle) transition temperature, and disposition towardsintergranular corrosion. These phenomena are ascribed tochromium-containing nitrideand carbide precipitations which take placeas a result of the too low solubility of carbon and nitrogen in ferrite.

Prior investigations have uncovered the fact that suchchromium-containing precipitations can be avoided by alloying the steelwith elements which bind carbon and nitrogen more strongly than chromiumdoes, for example with titanium or niobium. It has been found, however,that this treatment gives a material with inferior impact strength,probably because of the formed titanium or niobium carbonitrides.Furthermore, welding of such material is difficult as this easily causesprecipitation of chromium-containing particles which in its turn meanslowered corrosion properties. Finally, it can be mentioned that titaniumand niobium carbonitrides and oxide inclusions collect into streakswhich cause surface defects in the material, particularly after grindingor polishing.

By drastically decreasing the contents of carbon and nitrogen far belowthe normal level, a ferrtic chromium steel is obtained which has verygood corrosion properties and a low transition temperature. It should bementioned that a normal carbonand nitrogen content is in this case:

150 I500 ppm 200 500 ppm where l ppm =0.000l% by weight.

In chromium steels with said improved properties the total contents ofcarbon and nitrogen have to be reduced to 100 ppm or lower. Such steelshave been introduced lately. They have the following compositions:

Cr Ti.Nb C+N l8 28 of O 3 may be present very low Also for austeniticstainless chromiumnickel steels ex tremely low levels of carbon andnitrogen are of interest. The corrosion properties are improved and theductility is increased, which among other things means a betterworkability. It is, however, associated with great difficulties toproduce steels with total carbon and nitrogen contents of about or below100 ppm in the presence of a high chromium content. It may for examplebe mentioned that the solubility of nitrogen in iron, free fromchromium, is 500 ppm, while iron with 70 Cr can dissolve up to 5,000 ppmnitrogen.

Therefore, special methods have been tried in order to successfully meetthis situation. Such a method is electron beam refining of a chromiumsteel, in a high vacuum. upon a series of watercooled hearths. This wayis complicated; it has high operating costs and dcmands greatinvestment. In a vacuum induction furnace a very low carbon content,l0-2U ppm, can be reached by a lengthy vacuum treatment. The content ofnitrogen remains, however, usually at levels above lUU ppm. By combiningvacuum induction and vacuum arc remelting (VAR) or double VAR a sum ofcarbon and nitrogen below ppm has been obtained. This is also a veryexpensive method, however.

In a conventional arc furnace lower contents than about 250 ppm carbonand 250 ppm nitrogen cannot be practically produced in the presence ofchromium. If a process step including vacuum decarburizing with oxygengas is followed after the procedure in the arc furnace, a low carboncontent of about 50-80 ppm can be reached, but the nitrogen contentremains at l50200 ppm. Similar conditions apply to the AOD- process(argon-oxygen-decarburizing), in which the chromium-containing charge isdecarburizcd by means of a mixture of oxygen and argon. Remelting ofchro mium and iron with low contents of carbon and nitrogen in an inertatmosphere in, for example, a HF-(high frequency induction) furnace isimpeded because available chromium raw materials such as ferrochromiumhave too high contents of nitrogen and carbon.

From the above-mentioned technological findings it is obvious that theremoval of nitrogen is a greater problem than that of carbon refining.The carbon is removed by oxidation to carbon monoxide, while nitrogen isremoved by the reaction: N (dissolved) V: N (gas). This reaction is veryslow, kinetically considered.

In an earlier known process the nitrogen problem is solved in thefollowing way. Chromium pig iron produced in a cupola furnace isdecarburized in an LD- converter to about 0.3 C, after which the melt isvacuum-oxidized to the above-mentioned low content of carbon. A lownitrogen content of less than 100 ppm is reached thereby that thecarbonand silicon-rich chromium pig iron having a relatively lownitrogen content is refined from nitrogen by the very vigorous evolutionof gas in the LD-converter during a decrease of the carbon content from5 to 0.3 percent. The method pre-supposes, however, the opportunity ofusing special equipment. In the known process the low contents of carbonand nitrogen have been obtained at a Cr content of about 18 percent,thus a rather low content of chromium.

By the present invention a low nitrogen content is obtained by usingstrong nitrideformers to separate the nitrogen from a melt. Thisnitrogen-impoverished melt is then refined with respect to carbon andfinished to steel in, for example. an AOD-converter.

In the following, the invention will be discussed with reference to theappended drawing, in which FIG. 1 is a diagram showing equilibria of Aland N in aluminum nitride formation; and

FIG. 2 is a diagram showing influence of lithium in cooperation withaluminum in achieving a low content of nitrogen.

Studies of the free energy in forming nitrides from pure components showthat titanium and zirconium are stronger nitride formers than is, forexample, aluminum. Attempts to decrease the nitrogen content to very lowvalues by means of Ti and Zr fail in practice. however. The interactionbetween nitrogen and added nitride former thus affects in such a waythat Ti and Zr, but not Al, increase the solubility of nitrogen. In thedi agram of FIG. I, there is shown the equilibrium be- Curve C "7: Cr 2Temp. C

A 0 I535 B 0 25 I500 C 2 25 I300 D 2 25 I500 The diagram in FIG. 1 isthus specifically constructed for the purpose of explaining the inventedmethod, and it shows that a low content of nitrogen in the presence of ahigh content of chromium ought to be obtained by an addition aluminum toa melt containing carbon at a low temperature, i.e., a temperature nearthe melting temperature. As an example, according to the equilibrium 5A] at 1,300 C. corresponds to 2 ppm nitrogen in a steel melt with 2 Cand Cr. The importance of chromium is shown in the curves A and B. Itmay be mentioned that in case of titanium, the dissolved N- contentfirst decreases with increasing Ti-content, but already at a Ti-level ofabout 1 with the equilibrium content of N in the melt again increases.

The reason for non-observance of the possibility ofnitride-precipitation with aluminum may besides an insufficienttheoretical foundation be that a relatively high carbon content and alow temperature are essential. (Compare curves B and C in the diagram,FIG. 1. The nitrogen content according to the equilibrium is 50 timeslower on the curve C compared to B).

In the following text. specific examples will be given of nitrogenrefining according to the invention.

EXAMPLE 1 In a 50 kilogram open HF-furnace iron, FeCr and FeMo weremelted to the following analysis:

N 298 ppm Fe remainder. besides normal impurities.

After a temperature of 1.350C. had been adjusted, Almetal correspondingto 25% Al in the melt was introduced. After homogenization of the melt,test samples were taken which. upon analysis. showed a nitrogen contentof 93 ppm.

EXAMPLE 2 The same charging was used as in the foregoing example.Starting analysis:

Continued N 309 ppm Si 0.30 9; Mo 1.03 9;

Al-metal corresponding to 5.0 percent was added at 1.350" C. Aftermixing. a sample taken from the furnace showed 57 ppm nitrogen.

EXAMPLE 3 The same material was used as in examples I and 2. Startinganalysis:

C 157 7( Cr 25.8 if N 289 ppm Si 0.34 it Mo 1.00 9;

Al-metal corresponding to 9 percent was added at 1.350 C. The nitrogencontent was 40 ppm in samples taken from the furnace. From the examples[-3 it is obvious that the nitrogen is reduced with increased additionsof Al. See also the diagram in FIG. 2.

EXAMPLE 4 The same test as in the examples l, 2 and 3 was repeated butin a 200 kilogram open HF furnace and at a carbon content of about 3percent. Starting analysis:

C 2.70 7r Cr 25.2 /r N about 294 ppm Si 0.5l 71 Mo 0.95 71 Al-metal wasadded corresponding to 9.0 7r Al at l,300 C. In samples taken from thefurnace after homogenizing the melt, the nitrogen content was 45 ppm.Thus, a higher content of carbon had not caused any lower nitrogencontent than in the compared case. example 3.

EXAMPLE 5 The influence of the chromium content was examined. A chargewas produced in a 50 kilogram open HF furnace. Starting analysis:

At the temperature of 1.350 C., Al-metal was added corresponding to 5.07( Al. A sample taken from the furnace after mixing showed 30 ppmnitrogen. The example illustrates, in a comparision with example 2, thatthe refining is facilitated at a lower chromium content.

The examples show that a strong decrease in the nitrogen level isobtained by the help of aluminum. The contents are, however. far abovethe equilibrium values according to FIG. 1. This might depend uponincomplete separation of the nitride particles from the melt. For thisreason the following test was made:

EXAMPLE 6 Four charges of the same kind as in examples 1-4 were preparedin a 50 kilogram open HF furnace. The starting analysis of the meltsaveraged:

C ll Pi Cr 25.7 i N 320 ppm Si 0.4 F; Mo 1.05 01' Fe remainder At thetemperature of 1 ,350C. in the four tests quantities of analuminum-lithium alloy consisting essentially of 5 Li and 95 "/1 Al wasintroduced into the melt to a content corresponding to 1.0, 2.0, 2.5 and8.0 Li. After mixing the melt, nitrogen test samples were taken. Theanalysis showed 80, 45, and 20 ppm nitrogen (in the above order) afterthe increasing addition of Al and Li. compared to pure aluminum,aluminum-lithium gave a lower content of nitrogen, see the diagram FIGv2. This diagram shows the N content as a function of added content of Alrespectively LiAl. The upper curve sums up the results in the examples[-3, while the lower curve gives the result of example 6.

EXAMPLE 7 Pure lithium was added in two charges to control the effect inthe foregoing example. The amounts were 0.05 and 0.20 Li, respectively.The content of nitrogen before the addition was 285 ppm. After additionof Li the content was 250 ppm nitrogen. No important effect was thusobtained by an addition of Li as such.

EXAMPLE 8 The influence of changing from an iron-base to a nickel-basealloy was examined. A charge was produced in a 50 kg HF furnace.Starting analysis:

N 287 ppm Si (H8 "/1 Ni remainder besides normal impurities Al-metalcorresponding to 5 percent was added at l,350 C. A sample taken from thefurnace showed 43 ppm nitrogen.

EXAMPLE 9 In an open 50 kg HF furnace a chromium-nickel steel was meltedwith the following analysis:

Ni l0.2 5%

N 327 ppm Si 0.48 if Fe remainder besides normal impurities At thetemperature l350 C. Al-metal corresponding to 572 was added. A sampletaken after homogenization of the melt showed 46 ppm nitrogen.

EXAMPLE l0 In a 5000 kg arc furnace a melt was produced. Startinganalysis:

C 2.68 Ct Si 0.54 Ct Mo 1 IO "/5 Fe remainder besides normal impuritiesThe melt was tapped into a ladle where Al-mctal corresponding to6percent was added A sample taken after addition of aluminum showed 35ppm nitrogen and 5.1% Al. In order to remove aluminum and carbon themelt was transferred to an ADD-converter where aluminum and carbon wereoxidized. The analysis in the converter after this period was:

C 35 ppm Cr 22.1 7r

N 47 ppm Al 50 ppm Example 8 shows the result when using a nickel basealloy. The example 9 relates to a Cr-Ni steel. The example l0 shows thefinal removal of C and Al which should be done in a larger charge andnot in a small 50 kg furnace. (Examples 2, 3 and 4 have iron asremainder.)

The given examples all relates to chromium steel. As nickel has anincreasing effect upon the activity of the nitrogen, it is self-evidentthat the invention is applicable also to chromium steels. It is alsoself-evident that other Al-alloys than pure Al and LiA] can be used, forexample CaAl and MgA].

in the practical performance of the invention the raw materials may forexample be melted in a normal arc furnace so that a carbon content of1-2 percent is obtained. Such a charge may be based upon cheap rawmaterials and also contain a high content of circulating internal scrap.The latter material has great economical importance. The melt is thentaken for example to an ADD-converter, vacuum furnace or the like, inwhich aluminum or aluminum-lithium is added to the melt. After that, thenormal process of removing carbon is carried out. An intermediate slagseparation preferably is made, because aluminum gives rise to greatamounts to slag during the oxidation. When A] is oxidized by oxygen gas,a great amount of heat is evolved. The metal bath may then be cooled bysubstituting chromium ore for the oxygen gas or by adding suitablescrap. By adding chromium ore the reducing properties of aluminum areused for supplying chromium in a cheap way and with good heat economy.

The nitrogen refining addition of Al or LiAl may also be performed in aladle. The metal must, however, be protected from nitrogen pick up fromthe air when the content of the ladle is taken to the unit for oxidationof carbon. If the ladle has a high freeboard, meaning that the level ofthe melt is far below the upper edge of the ladle, vacuum oxidation canbe done directly in the la die.

It is also possible to granulate or in other ways disintegrate thenitrogen-refined chromium alloy. Decarburizing can then be performed insolid state in known ways, for example annealing in vacuum. The obtainedcarbonand nitrogen-impoverished chromium alloy may then be meltedtogether with likewise carbonand nitrogen-impoverished iron under anargon atmosphere. for example. in an HF furnace.

From the results it is obvious that at least 1 percent, and preferablyat the least 3 percent. Al in the form of aluminum metal must be addedto the melt in order to make an essential reduction of the nitrogencontent possible (compare the diagram FIG. 2).

As has been mentioned earlier. the carbon content should be relativelyhigh in the melt bath when the addition of Al is effected. Carbon has afavorable inlluence, as it in itself decreases the solubility ofnitrogen as well as lowering the melting point of the melt bath. Asuitable content of carbon has shown to be at least 1 at C.

The invented method is particularly applicable to melts containingbetween and 7: Cr. At actual contents of Cr and C the melt should have atemperature between l 300-l ,500 C.

The above-described method and the specific examples shown havegenerally discussed the conditions for removal of nitrogen and carbonfrom a chromiumcontaining base melt of iron. It is. naturally, withinthe scope of the invention to effect corresponding operations withchromium-containing base melts of other metals for which theprerequisite conditions are similar.

We claim:

1. Method of producing a chromium-containing alloy having very lowcontents of nitrogen and carbon not amounting, in toto, to more than0.01 percent by 8 weight. which comprises melting a charge containing atleast 1 percent by weight of carbon and consisting essentially of achromium-containing initial material together with a base metal anddesired alloying elements;

adding to the melt a nitridable member of the group consisting ofaluminum and aluminum alloys. in an amount corresponding to at least 3percent by weight of said melt. thereby converting nitrogen to aluminumnitride in the form of a constitutent of the resulting slag; and

thereafter refining the molten charge by oxidizing carbon and residualaluminum and removing slag containing aluminum nitride and aluminumoxide.

2. Method according to claim 1. wherein the melt has a content of Crbetween 15 and 30 percent.

3. Method according to claim 1, in which the melt is heated to atemperature between l,300l ,500 C. before Al is added.

4. Method according to claim 1, wherein the added aluminum is in theform of an alloy consisting essen tially of Al and a member of the groupconsisting of Li, Ba, Ca and Mg.

5. Method according to claim 4, in which carbon and aluminum remainingin the melt after the nitrogen refining operation are removed from themelt by oxidation.

6. Method according to claim 5, in which the oxidation of carbon andaluminum is effected with the aid of added chromium ore or scrap.

1. METHOD OF PRODUCING A CHRONIUM-CONTAINING ALLOY HAVING VERY LOWCONTENTS OF NITROGEN AND CARBON NOT AMOUNTING IN TO TOO MORE THAN 0.01PERCENT BY WEIGHT, WHICH COMPRISES MELTING A CHARGE CONTAINING AT LEAST1 PERCENT BY WEIGHT OF CARBON AND CONSISTING ESSENTIALLY OF ACHRONIUM-CONTAINING INITIAL MATERIAL TOGETHER WITH A BASE METAL ANDDESIRED ALLOYING ELEMENTS; ADDING TO THE MELT A NITRIDABLE MEMBER OF THEGROUP CONSISTING OF ALUMINIUM AND ALUMINIUM ALLOYS, IN AN AMOUNTCORRESPONDING TO AT LEAST 3 PERCENT BY WEIGHT OF SAID MELT THEREBYCONVERTING NITROGEN TO ALUMINIUM NITRIDE IN THE FORM OF A CONSTITUENT OFTHE RESULTING SLAG; AND THEREAFTER REFINING THE MOLTEN CHARGE BYOXIDIZING CARBON AND RESIDUAL ALLIMINUM AND REMOVING SLAG CONTAININGALUMINUM NITRIDE AND ALUMINUM OXIDE.
 2. Method according to claim 1,wherein the melt has a content of Cr between 15 and 30 percent. 3.Method according to claim 1, in which the melt is heated to atemperature between 1,3001,500* C., before Al is added.
 4. Methodaccording to claim 1, wherein the added aluminum is in the form of analloy consisting essentially of Al and a member of the group consistingof Li, Ba, Ca and Mg.
 5. Method according to claim 4, in which carbonand aluminum remaining in the melt after the nitrogen refining operationare removed from the melt by oxidation.
 6. Method according to claim 5,in which the oxidation of carbon and aluminum is effected with the aidof added chromium ore or scrap.